Revisions and rewrites. How do you know when it's time to stop?
At any given time, there are endless scenes running through my head. But, with all the variables and choices, the direction the scene takes can ultimately change the course of the entire story. That's one of the things that will stop me from tinkering with content beyond massaging and manipulating the wordplay - especially if the body of the material is complete.
But there is the idea that a story is never finished until publication - and then only because you've passed the point of no return and it's no longer in your control. For control freaks, this is a very difficult hinge point - finally letting go.
Thankfully, I don't suffer that malady. What you can do however, is save those scenes - additional and otherwise, for side stories and story-branch work if you're lucky enough to be writing a series like I am. And with multiple story lines within my work I can often find room to fit in something relevant to the overall body of work. But it has to add something; whether it be a wider picture of the peripheral world, or whether it's a piece of the woven storyline. In the end, it all has to mesh properly.
I don't schedule a predetermined number of revisionary passes for any of my work. Rather, I have a tendency to reread and rewrite as I progress, occasionally going back through the body 2 to 3 chapters back from where I'm currently writing. Right or wrong, I find it helps both in maintaining continuity as well as maintaining story flow. I want to get each story as perfect as possible but perfection is subjective and you have to be confident enough to know when done, is actually done. You don't want to be working on the same thing forever. But then again, if something feels off, you can't be afraid to address it.
If you already have readers or followers, use them as a sounding board - I certainly do. Their input can be critical, as a fresh set of eyes and fresh perspective can give you insights that are invisible when you are too close to the work. They can also give you options that you may not have devised on your own.
It all boils down to trusting your gut. You can't let your insecurities override your sense of reason when you're fairly sure you're done. You have to be able to objectively look at the reasons you're wanting to tinker. Is there something really wrong? Or are you afraid to let go? Writing and publishing has a steep learning curve and you have to let it go... to see if it will fly or fall. Then you can ultimately learn from your success or failure.